Forming wires for use in paper-making, cellulose and similar machines usually are in the form of a fine-mesh cloth which has been woven endless or otherwise joined into an endless web. As the very basis of good quality paper resides in the web formation itself, the structure of the forming wire is of vital and decisive importance. Until the middle of the fifties, all forming wires were manufactured from metal wires. These metal-wire cloths were useful in all kinds of paper-making machines and for all paper qualities. Around the date mentioned, metal-wire cloths, above all in cellulose machines, were replaced by single-layer cloths or wires of synthetic fibre threads, the so-called synthetic wires. The advantage of synthetic wires beyond metal-wire ones primarily resides in their improved wear resistance. Single-layer synthetic wires do, however, suffer from the disadvantage of having considerably higher elasticity and less stability than cloths made from metal wires of corresponding coarseness. On the large majority of up-to-date paper machines single-layer synthetic wires cannot be used at all, or only with difficulty, because of the large size and requirements on wire stability of such machines. Although considerable improvements have been made during recent years, only modest success has been achieved with single-layer synthetic wires on machines for e.g. wide and high-speed news-print paper, magazine paper and the so-called tissue paper machines. Also in the case of wide liner, kraft, and sack paper machines, several attempts have failed--despite the use of coarse and thus more stable single-layer synthetic wires.
The so-called double-layer synthetic wires consisting of two layers of one yarn system and a second yarn system interconnecting these layers have, as a result of their higher stability, considerably better chances of success on all types of paper-making machines, which several record runs also have evidenced.
A single-layer forming wire consists of two yarn systems only, the warp and the weft, whereas a double-layer wire must comprise at least three yarn systems. To interweave these yarn systems into a cloth possessing the same even surface structure as a single-layer cloth has hitherto caused the manufacturers large problems. The more complex binding structure of the double-layer wire involves marking problems, in that the structure of the yarns and/or the irregular mesh size leave traces in the paper sheet in the form of a so-called wire marking. The first double-layer synthetic wires had a geometrical structure that made it impossible in practice to bring to a common plane the two yarn systems closest to the material to be formed. The difference in levels between the knuckles of the warp and weft yarns caused such a pronounced marking that these wires were useful only in forming coarse paper qualities.
A considerable improvement is offered by the invention described in the Swedish Published Specification No. 366,353. The structure described therein makes it possible to locate the weft threads of the layer which in position of use of the wire faces the material to be formed, essentially tangentially to the wire plane facing said material. The invention provides a double-layer structure which is useful not only for coarse paper qualitites but also for the manufacture of e.g. newsprint paper.
In the manufacture of magazine and fine paper, the demands that the wire causes no marking are very high, among other reasons because the slightest tendency of the wire to cause marking, affects the printability of the paper. The wire structure described in the Swedish Published Specification No. 366,353 in several respects has proved suitable for use on many magazine paper machines, but its existing weak marking tendency, noticeable particularly diagonally with respect to the direction of travel of the wire and the paper, has limited its usefulness in these positions.
In the wire structure described in this Published Specification, each warp thread is made to bind or interweave separately with the layer of weft threads which in position of use of the wire faces the material to be formed. In this manner, the outer face of the wire will comprise a large number of short warp and weft float lengths.
A further development of the structure according to the Swedish Published Specification No. 366,353 is described in the Swedish Published Specification No. 385,486. The invention in accordance with this patent application likewise concerns a structure wherein the interconnecting synthetic warp threads also interweave separately with the weft threads of the layer of weft threads which in position of use faces the material to be formed. It is characteristic of the structure in accordance with the Swedish Published Specification No. 385,486 that the weft yarn on the opposite side of the wire which side in position of use thereof faces the dewatering members and thus is exposed to abrasion, is located tangentially to a plane positioned beyond the plane which is at a tangent to the warp threads interconnecting the layers. Owing to this structure, the wear resistance is increased in that a proportionally larger portion of the wear on the wire is transferred to the practically unloaded weft yarns.